Cell Biology: Small Molecules and Cellular Processes

Cellular Composition and Function

• Small molecules, including water, account for 75-80% of living matter by weight and 75% of a cell's volume.
• These small molecules serve as substrates for many cellular reactions, including energy metabolism and cell signaling.
• Cells acquire small molecules through importation and synthesis within the cell.

Proteins

• Proteins are the most abundant and functionally versatile cellular macromolecules.
• Proteins are formed by stringing together 20 different amino acids in linear chains, with a defined sequence.
• The linear chain of amino acids folds into a complex shape, conferring a distinctive three-dimensional structure and function on the protein.
• Proteins have various functions in the cell, including:
  • Enzymes that accelerate chemical reactions
  • Structural components of the cell (cytoskeletal proteins)
  • Sensors that change shape in response to changes in the cell
  • Import and export of small molecules and ions (proteins embedded in the cell-surface membrane)
  • Hormones and hormone receptors that regulate cell function
  • Binding to specific segments of DNA to turn genes on or off

Nucleic Acids

• Deoxyribonucleic acid (DNA) is the cell's "master molecule" that carries coded information for making proteins at the right time and place.
• The three-dimensional structure of DNA is a double helix, first proposed by James D. Watson and Francis H.C. Crick in 1953.
• DNA strands are composed of nucleotides, which are often referred to as bases because they contain cyclic organic bases.
• Four different nucleotides (A, T, C, and G) are joined to form a DNA strand, with the base parts projecting inward from the backbone of the strand.
• Specific segments of DNA, termed genes, carry instructions for making specific proteins.
• Genes contain two parts: the coding region specifies the amino acid sequence of a protein, and the regulatory region controls when and in which cell the gene's protein is made.